This invention relates to frame-to-frame comb filters for separating the luminance (Y) and chrominance (C) components of composite television signals and more particularly to means for correcting signal distortion due to the occurrence of motion between successive video frames in such a comb filter system.
Electronic signals which are periodic in nature may be processed advantageously by storing replicas of the signal which are separated in time by the repetition period and then combining the stored replicas to enhance information content of the signal. For example, conventional television broadcast systems (and most video record/playback systems) are arranged so that much of the brightness (luminance) information contained in an image is represented by signal frequencies which are concentrated about integral multiples of the horizontal line scanning frequency. Color (chrominance) information is encoded or inserted in a portion of the luminance signal spectrum around frequencies which lie halfway between the multiples of line scanning frequency (i.e., at odd multiples of one-half the line scanning frequency).
Chrominance and luminance information can be separated and detail information may be enhanced by appropriately combing the composite signal spectrum. Known combing arrangements take advantage of the fact that the odd multiple relationship between chrominance signal components and half the line scanning frequency causes the chrominance signal components for corresponding image areas on successive lines to be 180.degree. out of phase with each other (so-called interlaced frequency components). Luminance signal components for corresponding image areas on successive lines are substantially in phase with each other (and comprise non-interlaced components).
In a comb filter system, one or more replicas of the composite image-representative signal are produced which are time delayed from each other by at least one line scanning interval (a so-called 1-H delay). The signals from one line are added to signals from a preceding line, resulting in the cancellation of the interlaced frequency components (e.g., chrominance) while reinforcing the non-interlaced frequency components (e.g., luminance). By subtracting the signals for two successive lines (e.g., by inverting the signals for one line and then adding the two), the non-interlaced frequency components are cancelled while the interlaced frequency components are reinforced. Thus, the luminance and chrominance signals may be mutually combed and thereby may be separated advantageously.
The foregoing combing process is possible because of the inherent high level of signal redundancy line-to-line in a typical video scene. If one considers a stationary picture, and a particular pixel of that scene, it will be appreciated that the level of redundancy of the signal representing that pixel on a frame-to-frame basis will be substantially one hundred percent. Thus, if combing were performed on a frame-to-frame basis, the percentage of residual luminance (chrominance) signal in the combed chrominance (luminance) signal will be significantly reduced over a line-to-line combed signal. More particularly, there will be, for example, no vertical detail accompanying the combed chrominance signal. By design, the chrominance signal of an NTSC composite signal is synchronized to have a 180 degree phase relationship from frame to frame permitting comb filtering on a frame basis. The luminance component may therefore be extracted by linear addition of the composite signal from successive frames and the chrominance component may be extracted by linear subtraction of the composite signal from successive frames. The response characteristic of a line-to-line comb filter has nulls or teeth spaced at line frequency or 15.734 KHz intervals while the frame-to-frame filter response has teeth at 30 Hz intervals. The closer teeth or null spacing of the frame comb gives rise to a more complete combing process than the interline comb.
In the frame-to-frame combing process, non-stationary objects give rise to distortions in the reproduced images. These distortions are due to incomplete cancellation of the luminance and chrominance components in the combed chrominance and combed luminance signals respectively (cross components) as well as an effective bandwidth reduction of the luminance signal in the regions of motion. Differences in scene content due to object motion or panning, that occur in the time frame of one-thirtieth of a second or faster are characterized as being visual motion of the scene content. The motion induced distortions created by the frame-to-frame combing process are two dimensional and are due entirely to interframe scene motion. The distortions are observable in both the horizontal and vertical directions in the plane of the image and are manifested as dual or phantom images in the reproduced scenes. The dual images are separated by an amount corresponding to the rate of motion and may be accompanied by incorrect hue at the edges of the moving objects.
In contrast, interline comb filters create one dimensional distortions which are due to vertical structure within a scene even if stationary. Distortions attendant the interline combing process are manifested as an effective lowering of the bandwidth of the signal representing vertical scene structure and a softening of vertical edges of the scene content.
It is an object of the present invention to reduce the objectionable effects produced by motion in a frame-to-frame combing system.